Apparatus for drying particulate materials



Sept. 27, 1966 B. B. ASHBY 3,274,698

APPARATUS FOR DRYING PARTICULATE MATERIALS Original Filed Nov. 1, 1965 2 Sheets-Sheet l FEED m 2 fl :3 m D T 59 m r: 0 Ir THERMOCOUPLE 48 couneusl SURGE TANK T. c. l

EXTRUDER LIGHT MATERIAL RE-BOILER EXTRUDER HEAVY MATERlAL-' INVENTOR.

BILLY B. ASHBY,

QZQMQZTMQ ATTORNEY- DRUM DRIVE) Sept. 27, 1966 B. B. ASHBY 3,274,698

APPARATUS FOR DRYING PARTICULATE MATERIALS Original Filed Nov. 1, 1963 2 Sheets-Sheet 2 w- 0: E 200 1 Lu 1 m l 2 w I80 I l- TEMPERATURE PROFILE OF 60 FT. I60 ROTARY DRYER TUBES WHEN CONDENSING MIXTURE OF METHANOL AND n-BUTANOL AT 20 PSIA.

0 IO 20 3o 40 so so DISTANCE FROM VAPOR INLET, FEET.

FIG. 2.

INVENTOR.

BILLY B. ASHBY,

WWM M ATTORNEY- United States Patent 6 Claims. (Cl. 34-48) This application is a division of Serial No. 320,898, filed November 1, 1963.

This invention relates to the drying of particulate materials. More particularly, this invention is an improved apparatus for drying particulate materials 'by the use of vapors in indirect heat exchange contact with the particulate materials being'dried.

The usual practice in drying particulate materials is to flow the materials to be dried through a dryer. An inert gas is normally introduced in direct contact with the particulate materials to sweep away vapor formed during the drying operation. Steam is flowed through the dryer in indirect heat exchange contact with the particulate materials to be dried. The steam condenses inside tubes running the length of the dryer. The material to be dried comes in direct contact with the outside of these tubes. Thus, the heat required for drying is given up by the condensing steam, flows by conduction through the tube walls, and is transferred by conduction and convection from the outside surface of the tubes to the wet material. However, with the use of steam, the condensing temperature inside the tubes and, thus, the tube surface temperature is virtually constant along the entire length of the dryer. The drying capacity of a dryer is a function of the temperature of this heat exchange surface. In general, the higher the temperature, the higher the drying capacity so long as fouling of the indirect heat exchange contact surfaces does not occur.

In many cases, the maximum temperature permissible is limited by the melting point of the material being dried. The melting point often depends on the wetness or percentage of volatiles in the material. The greater the liquid content of the material, the lower the temperature must be to avoid fouling. Since the material entering the upper end of the dryer in most drying systems is relatively wet and the material leaving the lower end is relatively dry, the maximum allowable temperature varies considerably along the length of the dryer. For example, in drying a hydrocarbon solvent from polypropylene, the maximum allowable temperature varies from 180 F. at the inlet to 260 F. at the outlet.

From the foregoing, it is clear that for certain materials the temperature must vary along the length of the dryer (being higher at the lower end) in order that maximum drying capacity may be achieved.

Briefly described, my new method for drying a particulate material utilizes an inclined tube dryer. The particulate material is fed through the dryer from the higher end. A mixture of vapors is fed into the dryer tubes from the lower end of the dryer in indirect heat exchange with the particulate material. Each of the components of the vaporous mixture has a different boiling point. The ratios of the components of the vaporous mixture are controlled as well as the condensing pressure of the vapors in order to control the temperature gradient along the tubes of the inclined dryer.

The invention, as well as its many advantages, will be further understood by reference to the following detailed description and drawings in which:

FIG. 1 is a simplified diagram showing the basic equipment;

FIG. 1A is a sectional view taken along the line lA-lA of FIG. 1; and

FIG. 2 is a typical tube-temperature profile when condensing a mixture of methanol and n-butanol at 20 p.s.i.a.

Referring to FIGS. 1 and 1A, an inclined tube rot-ary dryer including a rotating drum 10 is used to dry the wet, particulate material. A plurality of tubes 12 is mounted within the rotating drum 10. The tubes 12 extend longitudinally from upper tube sheet 7 to lower tube sheet 9 through a major portion of the rotating drum 10. The rotating drum includes an upper stationary head 11 and a lower stationary head 13.

The wet, particulate material is fed into feed hopper 14 and then into the inlet of rotating drum by means of a motor-operated screw extruder 18. The dried, particulate material leaves the dryer at exit 20. The dried, particulate material is conveyed from the outlet 22 by a motor-operated screw extruder 24. If desired, an inert gas can be used to remove the dried, particulate material.

The mixture of vapors containing components with difierent boiling points is fed into the lower end of the inclined dryer from a reboiler 26 by means of a vapor conduit means 28. The lightest component of the vaporous mixture tends to concentrate and condense in the upper end of the tubes 12 at a relatively low temperature, while the heaviest component tends to concentrate and condense in the lower end of the tubes 12 at a higher temperature. The condensate drains down the tubes 12 countercurrent to the vapor flow. The condensate is fed back to the reboiler 26 .by means of the condensate conduit means 30. Light material can be bled from the vent in the upper end of the tube sheet 11 through bleed conduit means 32 and bleed valve 34. It is then condensed in a small auxiliary condenser and stored for recycling to the reboiler 26.

The temperature gradient along the'length of the dryer is determined, among other things, by the volatilities of the components in the mixture, the length of the tubes 12, and the amount of light and heavy materials in the tubes 12. The temperature gradient can be controlled auto matically by:

1) Bleeding off light material from the upper end of the dryer;

(2) Adding light material to the reboiler; or

(3) Changing the condensing pressure.

Light materials can be bled off from the upper part of the inclined dryer by means of bleed line 32 controlled by bleed valve 34.

The addition of light material to the reboiler 26 is controlled by an automatically operated valve 36 in light material line 38. The valve 36 is operated by temperature controller 40. The temperature controller 40 is, in turn, connected by means of electrical lead 42, slip ring 44 mounted in the rotating drum 10, and electrical lead 46 extending to a temperature-sensing means such as a thermocouple 48 medially positioned upon and extending into one of the tubes 12. When the temperature detected by the thermo-couple 48 exceeds a predetermined set point, the valve 36 is opened slightly to admit light material to the reboiler 26. When the temperature drops below the set point, the valve 36 is closed.

The pressure of, say, 20 p.s.i.a. within the inclined tubes may be held relatively constant by controlling the steam rate to the reboiler 26. The steam is fed to the coils 50 of reboiler 26 by means of steam line 52 controlled by valve 54. A pressure control 56 is mounted on the re'boiler 26. The temperature within the tubes 12 of the inclined dryer is a function of the pressure of reboiler 26 which, in turn, is a function of the flow rate of the steam fed to the reboiler.

FIG. 2 is a typical tube-temperature profile obtained by flowing a mixture of methanol and n-butanol through a 60-foot rotary dryer at 20 p.s.i.a. Methanol boils at a temperature of 164 F. at 20 p.s.i.a. The boiling point of n-butanol at 20 p.s.i.a. is 259 F. The mixture of methanol and n-butanol condensed in the rotary dryer consists of 45 percent methanol and 55 percent n-butanol.

Mixtures of two or more components consisting of vaporous materials other than methanol and n-butanol can be used in practicing my new method. For example, a mixture of hexane and Z-methylheptane can be used instead of the mixture of methanol and nbutanol. methylheptane boils at 260 F. at 18 p.s.i.a.

The partciular mixture used in a particular drying operation and the condensing pressure depend, among other things, on the material to be dried and the temperature profile desired.

' I claim:

1. In an inclined tube heat exchanger having a rotating drum, upper and lower tube sheets within and depending from said drum, a plurality of tubes extending longitudinally within said drum and depending from said upper and lower tube sheets, upper and lower stationary heads sealingly connected to said drum for allowing rotative movement of said drum relative thereto, means for charging a particulate material into said drum through an inlet at said upper stationary head, and means for discharging said particulate material from said drum at an exit at said lower stationary head, the improvement of vapor inlet means in the upper portion of said lower stationary head,

condensate outlet means in the lower portion of said lower stationary head,

vent means in said upper stationary head,

reboiler means including heating means,

means for sensing the vapor pressure within said reboiler,

means responsive to said vapor pressure sensing means for controlling said heating means,

Hexane boils at 170 F. at 18 p.s.i.a., and 2- and vapor conduit means fluidly connecting said re- Y boiler With said vapor inlet means.

2. Apparatus in accordance with claim 1 further comprising means for sensing the temperature within one of said tubes,

and means responsive to said temperature sensing means for controlling flow through said bleed conduit means.

3. Apparatus in accordance with claim 2 wherein said temperature sensing means comprises thermocouple means medially positioned upon and extending into one of said tubes. 7

4. Apparatus in accordance with claim 3 wherein said means responsive to said temperature sensing means comprises a temperature controller and valve means responsive to said temperature controller for controlling the flow of fluid through said bleed conduit means.

5. Apparatus in accordance with claim 3 wherein said heating means comprises a steam coil.

6. Apparatus in accordance with claim 5 wherein said means responsive to said vapor pressure sensing means comprises a pressure controller and valve means responsive to said pressure controller for regulating the flow of steam through said steam coil.

References Cited by the Examiner UNITED STATES PATENTS 2,162,973 6/1939 Roland 34-134 3,039,201 6/1962 Esenwein et a1 3448 KENNETH W. SPRAGUE, Primary Examiner. 

1. IN AN INCLINED TUBE HEAT EXCHANGER HAVING A ROTATING DRUM, UPPER AND LOWER TUBE SHEETS WITHIN AND DEPENDING FROM SAID DRUM, A PLURALITY OF TUBES EXTENDING LONGITUDINALLY WIHTIN SAID DRUM AND DEPENDING FROM SAID UPPER AND LOWER TUBE SHEETS, UPPER AND LOWER STATIONARY HEADS SEALINGLY CONNECTED TO SAID DRUM FOR ALLOWING ROTATIVE MOVEMENT OF SAID DRUM RELATIVE THERETO, MEANS FOR CHARGING A PARTICULATE MATERIAL INTO SAID DRUM THROUGH AN INLET AT SAID UPPER STATIONARY HEAD, AND MEANS FOR DISCHARGING SAID PARTICULATE MATERIAL FROM SAID DRUM AT AN EXIT AT SAID LOWER STATIONARY HEAD, THE IMPROVEMENT OF VAPOR INLET MEANS IN THE UPPER PORTION OF SAID LOWER STATIONARY HEAD, CONDENSATE OUTLET MEANS IN THE LOWER PORTION OF SAID LOWER STATIONARY HEAD, VENT MEANS IN SAID UPPER STATIONARY HEAD, REBOILER MEANS INCLUDING HEATING MEANS, MEANS FOR SENSING THE VAPOR PRESSURE WITHIN SAID REBOILER, MEANS RESPONSIVE TO SAID VAPOR PRESSURE SENSING MEANS FOR CONTROLLING SAID HEATING MEANS, BLEED CONDUIT MEANS FLUIDLY CONNECTING SAID VENT MEANS WITH SAID REBOILER, 